The present invention relates to a cuvette for a spectrometric analysis equipment, said cuvette having two opposing windows made of a material, which is transparent to the light of the waveband used for the analysis, said windows defining a limited light path of a light beam passing through a cavity inside the cuvette. More specifically the invention relates to a cuvette used in spectrometric equipment such as interferometers, essentially for analysis of fluid samples. Further, the invention relates to a spacer for the cuvette and a method for producing the spacer.
Generally, presently used cuvettes apply parallel windows. A method and flow system for spectrometry and a cuvette is disclosed in DK patent application No. 1221/96 and in the corresponding WO 98/20338. A cuvette having a wedge shaped spacer is claimed in WO 98/20338.
The Problem
The present invention specifically relates to spectroscopic equipment for analysis of milk. Such equipment uses fairly thin cuvettes having light paths of typically about 15-50 xcexcm. Optical cuvettes used for high pressures and dedicated to have a long life in flow systems in which they are highly exposed to intensive wear may advantageously be provided with diamond windows. Due to considerable differences in refractive indexes between fluid in the cuvette and the diamond windows the light beam may be subjected to multiple reflections, i.e. the cuvette may become a resonator. Such reflections and resonance will influence the recorded spectra as explained further in the detailed specification. This poses a problem when it is the intention to produce and distribute a great number of IR measurement instruments. In this respect, it is essential to be able to transfer standard calibrations to such instruments. Otherwise, it would be necessary to perform individual calibrations on each of the instruments. Such calibrations are time-consuming. Accordingly, it is essential to be able to standardise all instruments, a process, which may be performed by measuring a standard fluid on the instrument, and adjusting the instrument to give a predetermined response. Experience has shown that multiple reflections occurring in the cuvettes having substantially parallel diamond windows obstruct the use of standard calibrations. Accordingly it is the aim of the present invention to provide a cuvette which can be produced in great numbers and with which the spectrometric equipment can be standardised regularly in order to compensate for gradual changes in the components. After exchange of a worn-out cuvette, the equipment must be readjusted followed by regular standardising of the equipment.
The invention relates to a cuvette for spectrometric analysis equipment, said cuvette having two opposing windows made of a material, which is transparent to the light of the waveband used for the analysis, said windows defining a limited light path of a light beam passing through a cavity inside the cuvette. According to the invention the cuvette is characterised in that the window surfaces forming the cuvette cavity are non parallel, thereby ensuring that the internal distances between opposed areas of the windows surfaces will vary across the transparent windows.
By these means it is ensured that the waves reflected internally in the cuvette cavity, will arrive in differing and substantially random phases, if they return to the transmitted main beam to be detected by the electronic detector means associated with the spectrometric equipment. In this way they will add to a sum of signals wherein tendencies to resonance are highly attenuated. Thereby the unwanted ripples in the spectrometric absorption diagram are avoided.
Preferably, one of the window surfacesxe2x80x94or at least one or more areas of the window surfacexe2x80x94is arranged oblique, i.e. forming an angle slightly different from 0xc2x0 with the other window surface. This is one way to ensure that the width of the cuvette cavity varies within a predetermined interval.
In a preferred embodiment according to the invention, the space between the two windows is wedge shaped. In the preferred embodiment according to the invention a wedge shaped spacer is arranged between and separating the two windows to define a slightly wedge shaped cuvette cavity. By these means, the unwanted ripples in the recorded spectra are almost avoided.
In an advantageous embodiment, at least one of the window surfaces is a slightly curved, e.g. a substantially spherical segment or almost spherical segment. The curved surface ensures that the width of the inner cavity varies with-in a predetermined interval.
Preferably, the material of the windows is diamond, Germanium, Zinc Selenium, Silicon or Gallium Arsenide. The windows may be coated by wear resistant coatings, such as hard carbon. The thickness of the spacer may vary from e.g. about 10 to 200 xcexcm. Generally, the preferred thickness will depend on the properties of the medium or fluidum to be measured. In the case of raw milk or a dairy product, the preferred thickness of the spacer is about 20-50 xcexcm.
According to the invention the thickness of the spacer may vary gradually xe2x80x9cacrossxe2x80x9d a diameter of the spacerxe2x80x94and along the periphery of the optical opening of the optical instrument. Preferably, the spacer is wedge shaped. The spacer may be made of several material, e.g. metal or plastic materials, depending on the conditions to which it will be subjected or exposed.
According to the invention a spacer may be produced in the following way: A sheet of metal film is submerged in an etching bath, such as ferric chloride, and the submergence depth is controlled and vaned by the time.
Preferably, the submergence depth is gradually increased until substantially the whole sheet is submerged and removed from the etching bath, or that substantially the whole sheet is submerged completely the submergence depth being gradually decreased until substantially the whole sheet is emerged and removed from the etching bath and that the submergence is time controlled. Preferably, the sheet of metal film has an original thickness of about 60 xcexcm.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.